One technique used in high-speed digital communications systems is known as packet switching and may be used for transmitting all types of data, including voice, for a wide variety of applications. Packet switching digital networks allow data to be transmitted to other channels in the networking system in a packetized form. These packets contain such information as destination network and channel, origin network node and channel, parity information, as well as the data being transmitted. Because voice is an analog signal, it must be digitized prior to being transmitted on the networking system. Such digitization is known as compression. Converting the digitized signal back to an analog form is known as expansion. Performing both of these functions is known as "companding."
An international standard for converting voice to a digital form for transmission in a network is know as Pulse Code Modulation (PCM). Samples of the voice are taken at 8000 times per second, and the height of the pulses are converted to digital values. PCM needs only 8 bits to measure pulse height over the full volume range of a voice. The specific form of non-linear encoding for PCM is the ".mu.-law" algorithm in North America and Japan, and "A-Law" in the rest of the world, particularly Europe. In North America and Japan type T1 standard channel banks are used in the networking systems which are capable of handling up to 24 incoming channels at a maximum rate of 1.544 megabits per second (Mbps). In most of the rest of the world, type E1 (CEPT) standard channel banks are used which are capable of handling up to 30 channels at a maximum rate of 2.048 Mbps (with two additional supervisory channels used for control of the networking system).
In order to provide the most flexibility in a digital packet-switching system, allowing communication between North America, Japan and the rest of the world, these networking systems must provide a means for translating the ".mu.-law" encoded signal to an "A-law" encoded signal, and an "A-law" encoded signal to a ".mu.-law" encoded signal. Various techniques provide a means for accomplishing this, specifically the techniques described in the article, "Companding Routines for the TMS31130/TMS32020" by Lou Pagnucco and Cole Erskine, which appeared in Chapter 5 of Digital Signal Processing Applications with the TMS320 Family: Theory, Algorithms and Implementations at pages 169 through 197 (Texas Instruments 1986). These techniques require the use of a digital signal processor and its corresponding hardware. The use of a digital signal processor, however, is an expensive and complicated solution.
Alternatively, a lookup table may be used to convert from A-Law to .mu.-law and vice-versa. Because eight bits are sufficient to encode a human voice in PCM form, such a table would only require 256 entries for each conversion required. This requires at least 256 entries of eight bits each, along with the corresponding hardware, in order to implement the lookup table approach. Unlike the approach taken with the digital signal processors, at least 256 entries of eight bits each per conversion is required. This approach does have the advantage, however, that digital signal processors need not be used. Since memory is generally cheaper than digital signal processors, the lookup table approach is generally cheaper.
Another problem sometimes encountered with networking systems is that transmission levels may vary on certain channels in a system because of varying equipment quality, or due to other equipment coupled in the system such as channel banks and echo cancellers. Certain transmission channels may also be particularly strong or weak and may vary if certain lines are used primarily for long-distance. Also, according to a user's particular aesthetic requirements, the levels on particular channels in the system may need to be increased or decreased accordingly. Lastly, some equipment emits PCM with gain already applied, i.e. the transmission level point (TLP) of the equipment is already higher than the nominal TLP in the system. Therefore, it is desirable to apply a loss to the signal level to obtain optimum network performance. For the purposes of the remainder of this application, "gain loss" refers to adjusting the level of a transmission, whether it is a gain or a loss.
In view of these problems, most networking systems provide a means for varying the power level of particular channels in the system to maintain an optimum transmission level point (TLP). This has been done in prior art systems by using a separate power level circuit usually situated in a place before PCM expansion takes place. This adds greater complexity and cost, along with the attending pitfalls of such complexity (e.g. greater maintenance).